Constructing a podium for the largest air rights project to date in Washington, D.C., is one thing. Doing it in a confined work area over an active highway is something else entirely. Topping that, the project includes moving Washington’s oldest synagogue—twice.

The nearly 1,100-ft-long and 120-ft-wide concrete deck is just off Massachusetts Avenue and covers open-cut portions of Interstate 395. Completed in October, the deck spans a six-lane trench dividing Capitol Hill from the city’s East End that was built in the 1960s for a full inner loop highway system, halted because it would have bisected residential neighborhoods. The project also extends an existing on-ramp tunnel by 1,000 ft so it can more efficiently filter traffic to the freeways underneath the platform and improve pedestrian safety above it.

The seven-acre Capitol Crossing project is finally mending Washington, D.C.’s original street grid by creating three square blocks of real estate. The $1.3-billion multiuse development—including $270 million for utility upgrades and replacements and foundation work—will ultimately include five buildings containing 2.2 million sq ft of office space, apartments, green space, shops and restaurants.

Washington’s oldest synagogue was moved about 40 ft off the footprint of the development’s 750,000-sq-ft underground parking garage. Once the garage is complete, crews will move the synagogue back, atop the garage’s at-grade deck. Once that is done, work will begin on a Washington Jewish history museum, adjacent to the synagogue.

With an 11-year entitlement and permitting process and more than three decades of starts, stops and financial woes, Capitol Crossing is a complicated development, says Robert Braunohler, Property Group Partners (PGP) regional vice president. “The real challenge with a project like this is to have sufficient land value to justify the cost of building the platform, and there are only two cities where land value is high enough and that’s New York and Washington,” Braunohler adds. “There aren’t just empty sites just sitting around.”

Designing the foundation

Capitol Crossing’s $200-million deck is designed to withstand a 100-MW fire in the tunnel beneath it to meet federal highway requirements for fires and blast resistance put in place after the Sept. 11, 2001, terrorist attacks. The concrete deck was designed by the project’s structural engineer, LERA Consulting Engineers, based on recommendations from geotechnical engineer ECS.

LERA designed the deck with extra reinforcing steel in all the slabs to prevent disproportionate failure; if any one column fails, the deck and the buildings’ floor slabs are designed to survive.

The design team held at least three full-day charrettes with federal officials, blast experts and modeling experts to determine the design criteria for the deck and tunnel extension.

Foundation work was further complicated by poor subsurface conditions, says William Faschan, a partner with LERA.

Crews from Case Foundations installed drilled polymer slurry caissons, 8 ft to 10 ft in diameter with a 30-ft steel casing at the top. The caissons were cast up to 130 ft into the ground. An onsite slurry plant produced concrete mix for the caissons and the 867-ft- long and 3-ft-thick slurry wall. The wall is as tall as 76 ft.

The deck contains steel girders as long as 60 ft and as deep as 6 ft and concrete planks as long as 32 ft and as thick as 10 ft. The deck’s depth varied across the three city blocks and was limited to fit between the top of the highway right-of-way and the street grade.

To solve the challenge of long steel spans and limited deck depths, LERA considered various systems in concrete and steel. Ultimately, the engineer chose a system of composite structural steel and concrete box beams supporting 10-ft precast planks with 4-ft-deep composite cast-in-place concrete slabs. “Even in the temporary condition, the deck has to support heavy loads since the formwork and weight of the wet concrete of the superstructure needs to be supported during construction,” Faschan says.

The project’s complexity stemmed from balancing “the practical needs of construction and preserving design flexibility for future vertical development,” says Kristopher Takács, the Washington, D.C., practice leader in the office of architect-engineer Skidmore, Owings & Merrill. SOM provided master-planning, entitlements, infrastructure planning and urban design for the project. It is also providing architectural services for some of the buildings.

Designing the foundation with utility relocation and avoidance in mind, engineers had to accommodate a “spaghetti” of existing city utilities, some of which crews discovered during excavation, says Bob Robidoux, the purchasing director for the project’s general contractor, Balfour Beatty Construction D.C.

The utility design and planning also required onsite surveying and offsetting caissons from below their columns. The columns for the building superstructures are supported on transfer girders at the platform level, including a 63-ft-long and 4.5-ft-wide double-plate transfer girder that runs across the Massachusetts Avenue bridge that abuts the deck. The transfer girders are supported by columns, says Faschan, "which are, in turn, supported directly on caissons, one for one." Faschan added that there are no caisson caps and each column is constructed directly above a single caisson.

“Caisson dowels have tension couplers so the caissons could be paved over after installation,” says Faschan. “Each caisson had to be designed to independently resist all applied loads, including horizontal forces from accidental loading since it was impractical to install grade beams across multiple lanes of traffic to interconnect groups of caissons.”

The design team also coordinated its plan with the traffic maintenance plan as one side of the highway, either northbound or southbound, was always required to remain open.

In late 2014, workers from Balfour Beatty began relocating utilities and installing new ones needed for a new highway portal and the surface street grid redesign. This included storm and sewer lines, water lines, electrical and telecommunications duct banks.

Massachusetts Avenue could only be closed on Saturdays or Sundays during the nearly completed utility phase. If it rained on those days, “you were in trouble,” says Robidoux.

Long Nights

Around the same time utility work was getting underway, city officials rebuffed PGP’s proposal to close a portion of I-395 for more than a year during construction. The developer said closing the highway would allow completion of the project in about 20 months instead of the 60-month mandated deadline, which was part of the purchase agreement with the District Dept. of Transportation and Federal Highways Administration.

Balfour Beatty created a 3D digital sequencing model that ultimately convinced officials to allow the closure of one side of the highway from 9 p.m. for eight hours. “We had to be cleaned up and gone by 5 o’clock to open up for rush hour every single morning,” says Robidoux.

The nearly eight-month deck operation began with workers from Case Foundations Co. drilling caissons on the east shoulder and the median of the highway. They installed a slurry wall on the west shoulder of the highway, too. Then, Facchina Construction Co. formed and placed 48-in.-dia concrete columns on top of the caissons and slurry wall. L.R. Willson & Sons erected the steel girders and the precast wall panels at the highway median before Facchina set the precast planks on top of the girders.

Finally, crews cast the self-consolidating concrete in the steel box girders along with the concrete topping over the precast planks before Davenport sprayed on fire-resistant material to protect the steel.

The team installed the foundation and deck from north to south for ease of construction and to be able to deliver the deck’s northeast portion in time to build the first building.

“You are trying to move equipment in and out, move material in and out, get concrete trucks in and out,” Robidoux says. “We were trying to pour concrete at 4 o’clock in the morning. Once you start pouring a caisson you have to finish it, and getting concrete here at 4 o’clock in the morning [presented] some challenges.”

Used to lift girders, the project’s 300-ton lattice-boom truck crane was driven by the operator onto the highway worksite every night. The 26-ft-wide crane was parked all day just off the highway before it traveled a half mile per hour to its working position through an opening in the deck columns with a 3-in. clearance. “Every night for eight months they moved it in and out, in and out,” says PGP’s Braunohler.

Eco-district

The developer says Capitol Crossing is planned as the first “eco-district” in the nation’s capital. All five buildings are designed to qualify for a LEED Platinum rating. The buildings will have green roof areas and a water capture and containment system. Eight tanks—one 23 ft deep—will store groundwater from a riverbed beneath the project. The captured water will be minimally treated and used for cooling. The system is expected to save about $40% of the complex’s potable water intake.

A 2-MW cogeneration plant will generate electricity and heat. Two of the three generators in the plant will be fired by natural gas. The third will be powered by the heat generated from the other two generators. The system will reduce the tenants’ electric bills, Braunohler says.

Eco chimneys, made of large plants, will filter garage exhaust. An axial ventilation system was installed in the expanded highway tunnel with Saccardo nozzles that move air through the tunnel in the direction of traffic. The original portion of the tunnel previously used a traditional lateral ventilation system that blows air in the middle of the tunnel and sucks it out of vents.

With the new method, air moves southbound on one side and northbound on the other side. “This is a much safer ventilation system because if there’s an accident or a fire you are blowing the smoke away from the traffic that’s trapped” and out the far end of the tunnel, Braunohler says.

The new system required Balfour Beatty to modify and taper the tunnel walls to create concrete ductwork that channels exhaust out of the tunnel.

Years in the making

Local developer T. Conrad Monts acquired the air rights over the highway in 1989 but was embroiled in litigation with the city over land ownership. In 2005, PGP began negotiating to pay Monts the cost of a settlement so PGP could buy the property from the city at fair market value. After Monts died in 2009, PGP acquired the air rights for the project after paying an undisclosed amount to Monts’ widow to settle the lawsuit. PGP also worked out a deal with D.C. officials that uses a formula to determine how much it will ultimately pay the city for the property based on how much it cost to build the deck. The amount will be determined within 17 months, says Braunohler. PGP has already paid the city $9 million.

PGP also is paying up to $500,000 to move the 140-year-old Adas Israel synagogue and $9 million to the Jewish Historical Society of Greater Washington for its new adjoining museum, according to Braunohler.

The synagogue was originally moved three blocks in 1969 to Third and G streets to make way for the Metropolitan Area Transit Authority headquarters. In November, Wolf House & Building Movers transported the 273-ton brick structure about 40 ft to a temporary position.

Wolf House spent weeks setting up the move. First, crews detached the building from its foundation and used hydraulic lifts to install a framework of steel beams under the 24-ft by 60-ft structure. Then, on moving day, remote-controlled dollies slowly drove the synagogue about 1 foot per minute to a temporary location in a little less than an hour.

The synagogue will remain there for about two years while the Capitol Crossing garage is built. Then, crews will move it another 1,000 ft to its final location.

Balfour Beatty weatherized the synagogue and braced the windows. Two air-handling units will cool the interior during the summer, and weekly maintenance will be conducted.

The 103-year-old Holy Rosary Catholic Church also adjoins the deck at Third Street NW. Crews will demolish the church’s rectory before building a $10-million annex behind the church, on the deck. PGP says it will expand the church’s cultural center as well.

Meanwhile, on the opposite end of the deck, construction of the project’s first office building is more than half complete. The 12-story, 430,000-sq-ft structure is slated to open in 2018. The other four buildings should all be completed by 2022, Braunohler says.

“It’s an urban planning victory to be able to reconnect the city street grid and make a highway disappear,” Braunohler says, “and that’s really a great feeling to be part of that.”

Did you enjoy this article? Click here to subscribe to Engineering News-Record